This invention relates to a single acting hydraulic actuator with variable return force, which is suited for automatic control of safety valves and for rapid closure of such valves in emergency situations.
Such single acting hydraulic actuators are normally used on gate valves and are mounted directly to a valve stem which is movable for opening as well as for closing of the valve. An example of practical utilization in the oil industry is on the so-called Christmas Trees, as emergency blow out preventer valves for the main production stream of hydrocarbons, where the valves will be closed safely and efficiently when an abnormal situation arises. Such valve actuators are known for use both in sub sea and in top side equipment, for example on the deck of an offshore oil production platform. Such actuators will normally be mounted directly on the bonnet or housing of the valve, and the piston rod of the actuator will be connected to the valve spindle for control and regulation of the valve. As long as the control circuit is pressurized the valve rod or spindle is moved to and kept in the open position and a spring in the actuator is tensioned or loaded. When pressure is removed the pressure or force of the spring will be released and act via the piston rod directly on the valve spindle and move this against the frictional resistance of the valve to a position where the valve is completely closed.
The prior known valve actuators of this type, for example as discussed in U.S. Pat. No. 4,527,631 and the brochure "Model "OOH" Linear Hydraulic Gate Valve Actuator"--AVA-International--of Feb. 7, 1984, have direct connection between the valve spindle and the spring mechanism. This means that when the spring is extended 10 mm, the valve spindle will have moved a corresponding distance. This is impractical and inconvenient due to the fact that the demand for available closing power usually is highest when the spring is depressurized and near the end of its extension.
Furthermore, it is a requirement that the valve, in addition to securing efficient closure, also have the capacity to cut through any possible hindrances, such as tubes, wires and the like, that might be blocking the free closure of the gate valve within the bore or in tee tube string. This has resulted in the actuator equipment being voluminous and requires a great deal of space. Correspondingly, the dimensions of the spring are very great, the spring mechanism itself having an extended length of 1.5 m or more.